1. Field of the Invention
The present invention relates to wastewater treatment systems and, more particularly, to an improved drain field system that can operate passively.
2. Description of Related Art
Traditional sewage systems, such as those used for disposing waste from homes that are not connected to sewer lines, typically comprise a concrete, plastic, or steel septic tank into which both solid and liquid waste flow. The tank has one or more compartments through which the sewage flows horizontally and is kept out of contact with the air for a minimum of 24 hours. Spontaneous biological action liquefies much of the organic matter, while fine particles settle to the bottom, where bacteria convert some of the organic matter into methane and carbon dioxide. The solid matter either decomposes or is periodically pumped out of the tank.
The liquid flows out of the septic tank through a perforated pipe surrounded by loose aggregate, usually a bed of rock or gravel. The soil itself then continues the filtering process, and the liquid ultimately returns to the ground water.
The installation of such sewage systems entails digging a trench or bed into which is poured aggregate in the form of rock, crushed stone, or gravel. The perforated pipe is then laid down on the aggregate, and additional aggregate is added to a required depth. The top layer consists of soil cover, preferably planted, to facilitate surface water runoff.
Conventional systems require a considerable amount of skilled labor and expensive materials. The installations must meet stringent state and local codes, and must often take place in difficult terrain. For instance, suitable fill material is often difficult to obtain, since the aggregate must meet size and cleanliness requirements.
An additional problem with currently used systems is that the aggregate material, being of nonuniform sizes, has variable properties with regard to retention and evenness of distribution. Fines associated with the aggregate are capable of sealing off with sewage material, which prevents further filtration at such sealed off sites.
Another problem with conventional systems is that the depth of aggregate is 12 in., and with a typical 24-in. water table separation, the drainfield is 36 in. plus the cover, making many sites unbuildable or resulting in expensive upgrades.
The assignee of the present invention has also disclosed in U.S. Pat. Nos. 5,516,229 and 5,520,481 the use of assemblies of stacked void and distribution pipes in a plurality of configurations for use without aggregate. The disclosure of these patents is incorporated hereinto by reference.